Madison - If your tomatoes look tired and your pear trees seem peaked, you may find the source of the problem by looking up. If you discover a black walnut tree anywhere near your affected plants, you'll know your plants are being poisoned.

According to horticultural educators at the University of Wisconsin-Extension, black walnut trees release a chemical called juglone that causes leaves to wilt and turn yellow. Sometimes, it can kill the entire plant.

Juglone is released from walnut trees through the roots, through leaves that drop to the ground, through the husks of the nuts, and even from rain drops that drip from the tree's crown.

Several kinds of plants are very sensitive to juglone and should be grown away from a black walnut tree. Vulnerable vegetables include tomato, potato, pepper, eggplant, rhubarb and asparagus. Sensitive fruit crops include apple, pear, strawberry, blackberry, blueberry and domestic grape. Sensitive landscape trees include pine, birch, hackberry, silver maple, saucer magnolia, and basswood. Susceptible shrubs include lilac, potentilla, azalea, rhododendron, and viburnum. Among the most sensitive flowers near walnuts are lily, columbine and blue false indigo.

Juglone is not very soluble in water and it does not spread very far in the soil. The edge of the tree's crown, called the dripline, often has the most juglone. But the roots of a black walnut tree stretch as far as the dripline and sometimes twice that distance. In general, the toxic zone around a mature walnut tree is 50 to 60 feet of its trunk.

Horticultural specialists recommend you avoid planting black walnut trees in small yards. If you already have a black walnut tree on your property, grow the more sensitive plants away from the tree. Never use leaves, bark or wood chips of black walnut to mulch plants.

If you have a black walnut in a bad location, cutting down the tree won't solve the problem immediately. The juglone can persist until the roots are dead and decomposed, which can take five or more years.

Butternut, English walnut, pecan, shagbark hickory and bitternut hickory also produce juglone, but in smaller amounts.